Tracing hydrochemical water types and salinization mechanisms in the great Maputo area as a function of groundwater recharge, hydrogeological properties and human actvities

Guilherme Nogueira1, Tibor Stigter2
1 PhD fellow, Hydrogeology department, Helmholtz-Zentrum für Umweltforschung, UFZ - Leipzig
2 Senior Lecturer in Hydrogeology and Groundwater Resources, IHE-Delft

O 9.16 in Groundwater-surface water-interactions - processes and methods

22.03.2018, 17:00-17:15, 1

Groundwater resources in the Great Maputo area of Mozambique are under continuous pressure due to long droughts, scarce surface waters, inefficient public supply and increasing population growth. Locally high salinities further limit the exploitation of local aquifer, and it is important to study their origin, to correctly target mitigation and adaptation measures. The current study uses a combined hydrochemical-isotopic and multivariate statistical approach to improve the understanding of major factors controlling local hydrochemistry and the origins of inland high salinities in the multi-layered coastal aquifer system. Additionally, a Geographic Information System Multi-Criteria Decision Analysis (GIS-MCDA) is utilized to classify different recharge zones and potentials, based on hydrogeological properties and land use/cover, while recharge rates and periods are calculated through a root zone water balance method, enhancing the hydrosystem interpretation. Achieved results indicate recharge values of up to 30% of precipitation, with important contributions from extreme rainfall events. Higher recharge rates occur in areas with aeollian sediments covered by shrubland, while lower recharge rates occur in agricultural areas, largely due to high evapotranspiration rates. The analysis of hydrochemical and isotopic data allowed clustering of six water groups, varying from fresh (mainly CaHCO3, NaHCO3 and NaCl subtypes) to brackish/salt waters (mainly CaCl2, and NaCl subtypes). Inspection of water stable isotopes δ2H and δ18O (between -26.5‰ and +4.4‰, and -4.6‰ and +0.2‰, respectively) and Cl concentrations (0.2 ~ 200 meq/L), together with Na/Cl ratios (0.5 ~ 3.2, low in brackish/salt and elevated in fresh waters) and hydrogeochemical modelling suggest that evaporation and mixing with seawater are major processes defining salinity in the area, followed by water-rock interactions. Together with piezometric and geo-electrical data, δ18O/Cl ratios of brackish/salt groundwater (~ -4.0‰) and brackish/salt surface waters (~ -0.5‰) suggest that: 1) inland brackish/salt groundwater result essentially from mixing between infiltrating fresh waters and small fractions of seawater trapped within the clay layers, the latter possibly emplaced during last transgression periods and remaining of the system; and 2) brackish/salt surface waters result from seepage of brackish/salt groundwater into the river and associated wetland, followed by evaporation, hence increasing overall salinity and δ18O values. The source of (old) seawater, rather than halite dissolution, is further corroborated by Br/Cl ratios of brackish/salt water samples (1.2x10-3 ~ 1.4x10-3), near or just slightly below the ocean ratio (1.5x10-3). Cation exchange upon salinization is mainly observed in the semi-confined aquifer, while freshening takes place in the phreatic aquifer, especially in areas with high recharge rates, corroborating the identified salinization mechanisms in the area.

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